1. Field of the Invention
The present invention relates generally to an automatic gain control apparatus and method, and in particular, to an apparatus and method for compensating the gain of an automatic gain controller (AGC) in order to stabilize the received signal power of discontinuously transmitted high-rate packet data in a mobile communication system.
2. Description of the Related Art
Mobile communication systems for high-rate packet data transmission (hereinafter, referred to as a high-rate packet transmission mobile communication system) usually support only data channels or support both data and voice channels. The former is referred to as an International Mobile Telecommunication-2000 (IMT-2000) Evolution-Data Only (1xEV-DO), while the latter is referred to as an IMT-2000 Evolution-Data and Voice (1xEV-DV).
To realize high-rate data transmission, a plurality of users share the same channel for Time Division Multiplexing (TDM) in the high-rate packet transmission mobile communication systems. A base station receives feedback forward channel state information from a mobile station and if the channel state is acceptable, the base station transmits data in a high-order modulation scheme such as 8 Phase Shift Keying (PSK), 16 Quadrature Amplitude Modulation (QAM), or 64-QAM to achieve a higher data rate. Since a forward packet channel is shared among a plurality of users in TDM, the base station assigns all available transmission power to one or two particular users in a time slot assigned to the users.
Because packet data is usually generated discontinuously, there exist periods where packets are not transmitted e.g., non-packet transmission periods. Hence, if the received signal level of packet data is not kept constant, the use of a high-order modulation such as 64-QAM leads to poor packet reception quality. A typical solution is to use an AGC. The structure of the AGC and the level variations of a signal received at a receiver will be described below with reference to FIG. 1 to FIG. 2C.
FIG. 1 is a block diagram of a typical AGC for keeping a received signal level constant.
Referring to FIG. 1, the AGC comprises a gain controlled amplifier (GCA) 10 for receiving a signal s(t) from an antenna (not shown), an accumulator 20 for accumulating the output of the GCA 10 for a predetermined period, an adder 30 for adding the output of the accumulator 20 to a target reference voltage AIM_AMP, and a feedback loop filter 40 having a predetermined bandwidth, for filtering the output of the adder 30.
The GCA 10 is an amplifier controlled by a feedback signal.
During operation, an input signal is fed to the GCA 10. The output of the GCA 10 is divided into two parts and one of part of the output is fed to the accumulator 20. The adder 30 adds an accumulated signal received from the accumulator 20 to the reference voltage AIM_AMP with a negative value. That is, the adder 30 computes the difference between the output of the accumulator 20 and the reference voltage AIM_AMP. The feedback loop filter 40 filters the signal of the difference and the GCA 10 amplifies the input signal with the filtered signal.
The level variations of a signal received from a base station will be described in connection with the structure of the AGC.
FIGS. 2A, 2B and 2C are timing diagrams illustrating transmitted power level variations and received power level variations for discontinuous packet transmission. More specifically, FIG. 2A illustrates base station transmission power Ior, FIG. 2B illustrates the level of a received signal Îo—AGC controlled by the AGC, and FIG. 2C illustrates a control signal Vc(t) for controlling the GCA in the loop of the AGC according to the variation of the base station transmission power Ior. It should be noted that the base station transmission power Ior reaches a maximum value Pmax during a transmission period from t1 to t3, but falls to a normal value Pnormal during a non-packet transmission period.
However, the controlled received signal level Îo—AGC is not kept constant due to the rapid variation of the transmitted signal at the transmission start point t1 or the transmission end point t3 because the AGC generally operates in a loop control manner. Thus, some time is taken until the AGC loop is stabilized. It is an evitable AGC error under the assumption of an ideal AGC. Therefore, most high-rate packet receivers face the same problem in relation to discontinuous packet transmission.
High-rate packet data transmission involves a high-order modulation like QPSK/8-PSK or a higher-order modulation scheme such as 16-QAM/64-QAM. Demodulation performance is greatly degraded if the AGC error caused by discontinuous packet transmission makes the power level of an input signal inconstant.
Moreover, the AGC error lasting until the stabilization of the AGC during the discontinuous packet transmission changes the received power level in one slot, which significantly affects the demodulation performance of 16-QAM/64-QAM. Therefore, there is a need for an algorithm for reducing the power level variation of a signal output from the AGC in one slot.